This invention relates to transmission systems using multicarrier modulation. More particularly, the invention relates to multicarrier transmission systems that support multiple applications.
Digital Subscriber Line (DSL) technology provides high-speed transport of digital information over twisted pair phone lines. The typical DSL system uses a multicarrier modulation format that employs Fourier Transform as the modulation/demodulation engine. This transmission scheme is denoted as Discrete Multitone Modulation (DMT). The DSL system divides the transmission spectrum into multiple frequency bands called subchannels. Each subchannel can be modulated with a sinusoidal carrier to transmit information. A transmitter modulates an output data stream containing information bits onto one or more subchannels. A receiver demodulates all the subchannels in order to recover the transmitted information bits as an input data stream. For simplicity of reference, the term transceiver refers collectively to transmitters and receivers.
DSL systems can be used in an asymmetrical configuration (ADSL) where the data rate in a given direction, upstream from the residence to the central office and downstream from the central office to the residence, is different. DSL Systems can also be used in a symmetrical configuration where the data rate is equal in both directions.
The quality of transmission, in terms of Bit Error Rate (BER), can also be varied for different subchannels within the composite data stream. In addition to the high-speed digital transport over the twisted pair telephony wire, analog telephony, plain old telephone service (POTS), is also supported. The POTS signal occupies the low frequency region from 0-4 kHz and the digital data occupies the higher frequency band from approximately 30 kHz to many hundreds of kilohertz or several megahertz depending on the application.
ADSL enables a variety of applications, such as Internet access, digitized voice calls, video-on-demand, broadcast video, and video conferencing. Typically transceivers are designed and optimized for a single application because each application has different requirements for at least a) data rate, b) bit error rate, c) latency, d) symmetry/asymmetry of transmission and e) immunity to impulse noise and other transient phenomena. As a result, a transceiver that is optimized for one application, such as video-on-demand, does not work as well if used for a different application, such as Internet access.
Classic transceiver architecture includes a Framer/Coder/Interleaver (FCI) block, a digital modulation block, an Analog Front End (AFE) block, and a communications channel. In most cases, it is the FCI block that is optimized for a specific application because the FCI block has significant control over three of the four parameters mentioned above: bit error rate, latency and immunity to impulse noise. Many transceivers use forward error correcting (FEC) codes, block and/or convolutional codes, to improve the bit error rate (BER) performance. Combining large block FEC codes with interleavers provides immunity to impulse noise. The disadvantage of large block FEC codes and interleaving is that they add latency to the system. As an example, if low BER and immunity to impulse noise are required for a specified application, the transceiver may include FEC codes and interleaving. If for another application low latency is important but a higher BER and/or burst errors resulting from impulse noise are tolerable, convolutional codes and no interleaving may be used.
The Digital Modulation block, the AFE block and the transmission channel establish the data rate of the transceiver for a specified BER and margin. Advancements in signal processing techniques and silicon processes in the digital modulation block and AFE technology have led to significant improvements in the data rates achievable on twisted pair phone lines. The result of this dramatic increase in transmission bandwidth is the ability to transport multiple applications over a single transceiver connection. Given the different performance requirements of different applications, there is a need for a transceiver designed to transmit and receive for multiple applications. Furthermore, there is a need for a transceiver designed to dynamically switch from transmitting and receiving data for a first set of applications to transmitting and receiving data for a second different set of applications.
One objective is to provide a DMT transceiver that can support multiple applications and that can dynamically change the transmission and reception of data as a set of active applications changes. In one aspect of the invention a multicarrier system has a communication channel and features a method for supporting at least two applications.
Each application in a set of currently active applications is associated with a different Framer/Coder/Interleaver block for processing bits associated with that application. One application in the set of currently active applications is allocated a subchannel for carrying bits processed by the Frame/Coder/Interleaver block that is associated with that application. In response to a change in the set of currently active applications, the subchannel is allocated to a second different application for carrying bits processed by the Framer/Coder/Interleaver block that is associated with that second application.
In one embodiment, the number of bits carried on the subchannel allocated to the second application is changed with respect to the number of bits that were carried on the subchannel for the first application. In another embodiment, the subchannel that is allocated to the second application has a first subset of the bits allocated to the second application and a second subset of the bits allocated to at least one other application. In yet another embodiment the first application is an asynchronous transfer mode data application and the second application is a voice telephony application.
In still another embodiment, the step of allocating the one subchannel to the second application includes selecting each subchannel by ordering the subchannels based on the frequency of the subchannels and iterating through the subchannels from the lowest frequency subchannel to the highest frequency subchannel. In another embodiment, the step of allocating the one subchannel to the second application includes selecting each subchannel by ordering the subchannels based on the frequency of the subchannels and iterating through the subchannels from the highest frequency subchannel to the lowest frequency subchannel. In another embodiment, the step of allocating the one subchannel to the second application includes selecting each subchannel by ordering the subchannels based on the number of bits carried by the subchannels and iterating through the subchannels from the subchannel carrying the lowest number of bits to the subchannel carrying the highest number of bits. In yet another embodiment, the step of allocating the one subchannel to the second application includes selecting each subchannel by ordering the subchannels based on the number of bits carried by the subchannels and iterating through the subchannels from the subchannel carrying the highest number of bits to the subchannel carrying the lowest number of bits.
In still another embodiment, one of the applications in the second active application set is a voice telephony application having a plurality of bits including a set of ABCD signaling bits, and the ABCD signaling bits are processed by a different Framer/Coder/Interleaver block than the other bits of the voice telephony application. In yet another embodiment one of the applications in the second application set is a voice telephony application having a plurality of bits including a set of ABCD signaling bits where the ABCD signaling bits are processed by the same Framer/Coder/Interleaver block as the other bits of the voice telephony application.
In still another embodiment at least one of the applications in the first active application set is also in the second different active application set. In another embodiment at least one of the subchannels is allocated to at least two applications.
In another aspect of the invention, a multicarrier system has a communication channel and features a method for supporting at least two applications. Each application in a set of currently active applications is associated with a different Framer/Coder/Interleaver block for processing bits associated with that application. At least one subchannel is allocated to each application in the set of currently active applications for carrying bits processed by the Framer/Coder/Interleaver block associated with that application. In response to a change in the set of currently active applications, a previously unallocated subchannel is allocated to one application in the changed set of currently active applications for carrying bits processed by the Framer/Coder/Interleaver block associated with that one application in the changed currently active application set.
In another aspect of the invention, a multicarrier system has a communication channel and features a method for supporting at least two applications. The system processes bits associated with the one or more applications in a first active application set using a different Framer/Coder/Interleaver block for each application in the first active application set. The system allocates subchannels to one or more applications in the first active application set for carrying bits associated with the one or more applications in the first active application set. The system transitions and processes bits associated with one or more applications in a second different active application set over a different latency path for each application in the second different active application set. The system changes the allocation of subchannels to one or more applications in the second different active application set for carrying bits associated with the one or more applications in the second active application set.
In one embodiment of the invention, when the system changes the allocation of subchannels it changes the number of bits carried on at least one subchannel. In another embodiment of the invention, when the system changes the allocation of subchannels it reallocates at least one subchannel from one application to a second different application. In yet another embodiment, when the system changes the allocation of subchannels it allocates at least one of the subchannels to at least two of the applications in the second different application set.
In another embodiment, the first active application set includes an asynchronous transfer mode data application and the second different active application set includes the asynchronous transfer mode data application and a voice telephony application. In still yet another embodiment the first active application set includes an asynchronous transfer mode data application and a voice telephony application and the second different active application set includes the asynchronous transfer mode data application and excludes the voice telephony application.
In another embodiment when the system changes the allocation of subchannels, it at least allocates to the second different active application set a subchannel that was previously unused by one of the one or more applications in the first active application set. In yet another embodiment when the system changes the allocation of subchannels, at least one subchannel that was used by the applications in the first active application set is unused by the applications in the second different active application set.